Lead is persisting in the San Francisco Bay in
contrast to decreased concentrations in other nonestuarine environments,
say University of California, Santa Cruz (UCSC) scientists. Using lead
isotopic compositions, they found that lead from 1960s-1970s gasoline still
remains in the bay and at least 90 percent of late 1980s lead has yet to
enter the bay. Information about lead’s unexpected persistence may also
have implications for other contaminants like mercury, which have biological
ramifications.

“Even though we phased out leaded gasoline in
1992, we are still affecting the environment,” says Doug Steding, an earth
sciences Ph.D. student at UCSC. “We are seeing that, isotopically, there
is no change in lead over the last 10 years in the South Bay. The degree
of persistency is surprising,” Steding says.

Twice a year the University
of California, Santa Cruz, takes out the Research Vessel David Johnston
to help with sampling the bay's surfacewater. Photo
courtesy of Genine Scelfo.

Throughout the bay, lead currently suspended in
the water column and sediment is mostly from 1960s-1970s leaded gasoline.
More than 90 percent of late 1980s lead emissions is still in soils within
the river basins of the Sacramento and San Joaquin rivers and are gradually
being washed into the northern reach of the bay by winter rain. The hydraulic
mine sediments from the rivers have slightly diluted the relative amount
of 1960s-1970s gasoline lead in the northern reach.

“In the ocean, we can see declines in lead concentrations
that we can’t see in the bay because of the nature of the geologic setting,”
says Sam Luoma, a USGS hydrologist. Lead concentrations have decreased
in the ocean, atmosphere, polar snow and ice, and rivers in North America
since the phase-out of leaded gasoline, Luoma adds.

Steding, Charles Dunlap, a research fellow in
the Environmental Toxicology Department, and Russell Flegal, a professor
of Earth Sciences, Aquatic and Environmental Geochemistry, both of UCSC,
published their results in the Sept. 12 Proceedings of the National
Academy of Science. The 10-year-long study used samples taken from
the San Francisco Bay and the mouths of the Sacramento and San Joaquin
rivers.

Sediment was deposited in the bay until around
1950 when erosion became the dominant process, Luoma says. A large factor
in this change was the construction of dams in the river basins that essentially
starved the bay of sediment. Because the bay is shallow and wind and tidal
currents strongly affect it, sediment contaminated with lead is constantly
resuspended in the water column.

“Estuaries are usually thought of as sinks. But
in this case, it is a source — the sediment within the estuary is a source
for lead contaminants,” Steding says. The size of the bay’s mouth also
confines the sediment flow from the bay to the ocean. This natural limitation,
combined with human-induced changes in the hydrology of the area, make
the bay unable to efficiently reduce contaminants as effectively as other
environments. As a result, contaminants retained by hydrogeological conditions
affect the terrestrial and aquatic communities in the bay.

Lead concentrations in the bay are currently not
hazardous. However, other contaminants, like mercury, are exceeding suggested
limits and are a danger to the bay’s wildlife. “This research really gives
us a model for contaminants in the bay,” Steding says.

Mercury cycles through estuarine environments
in a similar way to lead. But scientists do not know the exact residence
time of mercury within the bay because, unlike lead, mercury does not have
a natural variability in its radioactive decay.

Isotopic data allow scientists to fingerprint
the nonpoint sources — such as gasoline from different decades in the case
of lead isotopes. Isotopic compositions, therefore, are better indicators
for how long contaminants stay within a system than are mass concentrations.
By knowing the degree of persistency of lead, scientists may gain a better
understanding of mercury’s potential impacts on the biological communities
of the bay. Researchers can use this model for other contaminants, such
as PCBs, chromium and nickel.